Erbium-doped optical fiber amplifiers (EDFAs) are well known, and have reached a high level of development. However, these amplifiers are limited to wavelengths of about 1.5 .mu.m. Indeed, at present there are no practical silica-based rare earth doped fiber amplifiers that can be used to provide gain at wavelengths of about 1.3 .mu.m, the operating regime of most currently operating optical fiber communication systems. See, for instance, S. V. Chernikov et al., Electronics Letters, Vol. 31 (6), p. 472, (March 1995).
Optical FRAs are known, and can be designed to operate at a desired wavelength at or near 1.3 .mu.m. See, for instance, S. V. Chernikov et al., op. cit. Indeed, Raman amplifiers are potentially promising candidates for such use because they can utilize silica-based fiber, and because of their high transparency when unpumped. For background information on stimulated Raman scattering see, for instance, "Nonlinear Fiber Optics", G. P. Agrawal, 2nd edition, Academic Press 1995, especially pages 16-19, and 316-335, incorporated herein by reference. See also U.S. Pat. No. 5,323,404, also incorporated herein by reference, which inter alia discloses FRAs with a multiplicity of optical "cavities".
Although it is relatively easy to generate large gains (e.g., small signal gain&gt;40 dB) through Raman amplification, prior art high gain FRAs generally are noisy, and we are not aware of any prior art FRAs that have both high net gain and low noise. However, the above referenced concurrently filed application discloses a counter-pumped optical FRA that can provide relatively high gain and relatively low noise. A significant feature of that Raman amplifier is the use of a multiplicity of amplifier stages, with an optical isolator between two adjacent stages. In view of the great potential of FRAs for use in optical fiber systems, e.g., as a replacement for conventional repeaters in 1.3 .mu.m systems, it would be highly desirable to have available such amplifiers capable of providing still higher gain at relatively low noise. This application discloses such amplifiers.
N. A. Olsson et al., J. Lightwave technology, Vol. LT 4 (4), p. 396 (April 1986) report measurements of the noise properties of a FRA, with signal-spontaneous emission beating being the dominant noise source. These authors also report (see p. 396, first column, first paragraph of section II) that use of counter-propagating pump radiation greatly reduces high frequency noise due to pump fluctuations.
A great deal of research has been done on the properties of EDFAs. For instance, S. L. Hansen et al., IEEE Photonics Technology Letters, Vol. 4 (6), p. 559 (June 1992) consider the limit placed on the maximum gain of EDFAs, and report observation of a significant amount of amplified Rayleigh backscattering (RBS), although the gain was limited by ASE saturation rather than RBS. They also report insertion of an optical isolator (and use of two WDMs to guide the pump radiation around the isolator), with attendant increase in the achievable gain. See, for instance, p. 561, last two paragraphs of "Discussion".
F. W. Willems et al., Electronics Letters, Vol. 30 (8), p. 645 (April 1994) measured the noise in EDFAs due to amplified double Rayleigh scattering, and found that attention to this noise source may be advisable for externally modulated analog AM-CATV systems. See, for instance, the "Conclusion" section.
S. Yamashita et al., IEEE Photonics Technology Letters, Vol. 4 (11), p. 1276 (November 1992) disclose that the noise and gain characteristics of an EDFA are improved by insertion of a "midway" isolator into the active fiber, with the optimal isolator position being 1/3-1/2 of the total active fiber length from the input length. See, for instance, p. 1278, last paragraph.
RBS in optical fibers is well known. J. L. Gimlett et al., IEEE Photonics Technology Letters, Vol. 2 (3), p. 211 (March 1990) disclose that RBS ". . . can be modeled as a "Rayleigh mirror" which in lightwave systems with optical amplifiers has the same effect as a single discrete reflection with an effective reflectance given by (R.sub.bs /.sqroot.2), where R.sub.bs is the backscattering reflectance", and that optical isolation may be essential for fiber systems using high-gain optical amplifiers. See, for instance, p. 213, last paragraph. The reported measurements were obtained on a system comprising 30 m of Er-doped fiber between long lengths (120 km and 18 km, respectively) of optical fiber.